Material coatings play an important role in our manufactured-products-based society. Coatings such as paints, lacquers, adhesives, resins, diamond films, and polymer films are used on many products used in both commercial and military applications. Material coatings will often improve the reliability, maintainability and quality of many products including microelectronics, optical surfaces, machinery, vehicles and standing structures. Such coatings provide immunity to corrosion, thermal insulation, as well as environmental shielding. In some applications the material coatings are required for very complex and specialized functions. These material coatings are selected because of specialized properties such as radio frequency absorbing properties, light absorption or reflection properties, and other predetermined optical properties. Many coatings are applied merely for aesthetic purposes such as appearance enhancement or item identification.
During the life of many manufactured products, such as bridges, aircraft, automobiles, and ships, painted coatings require removal and replacement for a variety of reasons. For example, refurbishment of the paint on aircraft is a regular maintenance item. Commercial airlines repaint their aircraft about every 4-5 years of service. The United States military typically repaints its aircraft after three years of service, or less. Coatings on the exterior surfaces of large ships or bridges require periodic refurbishment in order to prevent or inhibit corrosion. Specialized coatings may only extend the service life of an end item for several years before serious degradation and failure may occur.
The removal of paint from the surfaces of aircraft presents special problems. Aircraft surfaces are large, irregularly shaped, and relatively delicate. Because the surfaces of aircraft are typically lightweight aluminum or organically based composite materials, such surfaces and the underlying substrates are particularly susceptible to damage while undergoing paint removal that could degrade their structural integrity.
Many different methods have been used to remove painted coatings. One type, the particle medium blast (PMB) method, involves impinging the surface to be stripped with particles such as bb's, plastic media, steel shot, sodium bicarbonate, wheat starch, and/or sand. However, PMB methods energetic enough by themselves to remove hardened coatings such as paint may damage delicate surfaces. For example, if the nozzle supplying the impinging particles dwells too long at one location, the impinged surface may become pitted or stress hardened. This is especially important to avoid with regard to the surfaces of aircraft since pitting or stress hardening may change the mechanical properties of the surface material. High-energy PMB methods may also deform the surface of the substrate sufficiently to mask fatigue cracks and other anomalies that, if undetected and uncorrected, could lead to catastrophic failure of the substrate. PMB methods may also damage putty joints often found on aircraft between surface plates. Moreover, most PMB processes generate a large amount of particulate waste requiring costly disposal, since this waste is often contaminated by toxic constituents of the coating.
Another method involves the application of chemical agents to painted surfaces in order to chemically breakdown the layers of paint, thereby stripping the paint away from the surface to be exposed. However, such compounds may pose a risk to human health, are usually toxic, and often not biodegradable. Overall, these types of compounds are difficult and costly to dispose of because they present serious environmental problems. Government regulations are increasingly restrictive of the use of such agents.
Still other methods involving mechanical paint removal techniques have also been employed. For example, U.S. Pat. No. 4,836,858, entitled, "Ultrasonic Assisted Paint Removal Method" discloses a hand held tool, which uses an ultrasonic reciprocating edge placed in contact with the surface to be stripped. Unfortunately, employment of this tool is labor intensive and relies upon the skill of a human operator to use it effectively. Further, control of this tool is a problem when applied to aircraft because the aircraft surface may be damaged if there is excessive tool dwell at one location.
None of the aforementioned methods for removing paint are suitable for removing coatings from carbon epoxy surfaces. PMB and other mechanical grinding methods sufficiently energetic by themselves to remove paint have proven to damage composite materials. The removal of paint with chemical compounds does not offer a satisfactory solution because such chemicals tend to attack the composites, as well as the paint.
Radiant energy paint removal techniques are also known in various related art. U.S. Pat. No. 4,588,885, entitled "Method Of And Apparatus For The Removal Of Paint And The Like From A Substrate" discloses a method for removing paint that employs a pulsed laser directed to irradiate a target area of a painted structure to vaporize incremental layers of paint. In between laser pulses, a second light source illuminates the target area with light which reflects off of the target area and is received by a grating. The grating produces a spatial dispersion on a focal plane array. Electrical signals generated by the focal plane array represent the spatial spectral dispersion of the light and are used to control the output of the laser.
A problem with the system described in U.S. Pat. No. 4,588,885 is that it is very difficult to cover large areas, such as the surface of an aircraft, with the pinpoint beam of a laser. To do so requires sophisticated scanning and rastering techniques. Furthermore, a laser powerful enough to vaporize paint requires high power. Employment of such a powerful laser requires a large capital investment in order to provide space to operate the laser, as well as laser stops to prevent the laser beam from inadvertently escaping the work area and even the building where it is being used. Such a laser poses a serious danger to humans, who must be kept out of the area where the laser operates. Therefore, there is a need for a paint removal system that avoids the limitations of laser-based paint removal systems.
In addition, U.S. Pat. No. 5,281,798 and U.S. Pat. No. 5,194,723 each disclose a material removal process which uses a radiant energy together with a particle stream. In both disclosures, a preferred particle stream of carbon dioxide pellets is directed at the irradiated surface.
Another common problem with the related art systems which use radiant energy for removing paint and similar such materials is that some or all of the removed material is in the form of vapors which often deposit on the optical components of the system. Such a problem severely limits the usefulness of the equipment for any substantial length of time. Frequently, interrupting the paint removal process to clean the equipment involves additional costs due to the increased time and labor involved.
It should also be noted that the use of composite structures, manufactured, for example, of graphite epoxy or other reinforced plastic materials is becoming increasingly common. Many aircraft and automobiles extensively employ carbon epoxy materials for surface structures. Such structures are also painted or coated for a variety of reasons including aesthetics, identification, and camouflage. However, such painted and coated surfaces deteriorate under the action of weather and the mechanical forces to which they are subjected, thus requiring removal and replacement. A significant problem associated with removing paint or other coatings from these non-metallic substrates is the resulting damage to the composite substrate.
Other than hand sanding, there have been no suitable methods or apparatus for removing paint from non-metallic composite surfaces. PMB and mechanical grinding methods sufficiently energetic to remove paint by themselves have been proven to damage composite materials. The removal of paint with chemical agents does not offer a satisfactory solution because such chemicals tend to attack the composites, as well as the paint. Some composites are sensitive to high temperature, such that prior high temperature methods of removing coatings are not satisfactory.
Yet another problem associated with related art removal techniques is that many of the discussed techniques do not address the need to clean the substrate surface after removal of the coatings. Surface cleansing is often performed as a secondary or subsequent process to ensure that most contaminants, including PMB particle contaminants, are removed from the surface of the substrate. These secondary or subsequent processes involve expending additional time and labor.
For a variety of reasons, material removal techniques for removing paint from large surfaces, and surfaces having topological anomalies such as rivets, have not proven wholly satisfactory. Thus, it can be appreciated that coating removal, and particularly, the removal of paint from large and often delicate surfaces such as found on aircraft and automobiles, is a problem that has not been satisfactorily solved.